Spectrophotometer typically operates by passing a beam of energy from a source through a gaseous mixture to a detector. The energy beam, which may be infrared, visible light, or any other suitable wavelength, is chosen in accordance with the absorption wavelength or wavelengths of a gas or gases of interest in the gaseous mixture. By detecting or measuring the absorption at each selected wavelength, the presence or concentration of the gas of interest in the gaseous mixture may be determined.
In certain types of spectrophotometer, a sample cell is used for containing a selected volume and path length of the gaseous mixture to be analyzed. Where the gas is to be analyzed on a continuing basis, for example automotive exhaust gas, the sample cell will contain provision for passing the gas of interest into and out of the cell in a desired flow pattern. One example of a sample cell of this type is shown and described in U.S. Pat. No. 4,177,381, McClatchie et al., assigned to the assignee of the present invention.
It has been found advantageous, for certain applications, to reduce the size of the sample cell. By doing so, the volume of gas analyzed is reduced and certain advantages accrue, including a reduction in the pumping capacity required for the gas. One way of reducing the sample cell size is to make the interior of the sample cell frusto-conical in shape. An example of such a cell is shown in U.S. Pat. No. 3,887,473, Sternberg et al.
When a so called conical sample cell is employed in a spectrophotometer, certain problems may be encountered. For example, energy radiated from the source and reflected off of the interior walls of the sample cell may reach the detector. Such reflections have a different path length than radiation reaching the detector directly from the source. Accordingly, aberrations in the reading of the spectrophotometer may be introduced. Another problem may be as a consequence of condensate building up within the sample cell and collecting at the large diameter end of the cell. Without removal, this condensate could result in a erroneous spectrophotometer reading.